Unless you qualify for physician assisted suicide, like in Canada or Switzerland, you won't find anything.
The irony is when you want to die from overdosing, you live, but when you want to live, you end up dying from an accidental overdose. Even then, the person who dies usually has other health problems or is really old and already on their death bed.
That's why I check on the ages of people who I read about online who have killed themselves. If I see they were in their 80s, I know I cannot use them as an an example or source for my own method. These are outliers and people who were already dying.
Nembutal really is the only sleeping medication I've heard of that will work, and you won't find it on a black marker or dark web. You would have to go to Switzerland and qualify for death with dignity, which is expensive and very hard to do.
Suicide is painful, there's no question about it. Even with a gun, you have to be so far gone that you don't even have any SI to stop you from pulling the trigger.
However, there are options like inert gases that supposedly are painless. I tried to end my life with argon but failed. I used a homemade exit bag and while nervous didn't feel any SI from stopping me from pulling the bag over my head.
The problem was I didn't pass out. That was my only option, and I was devastated that it didn't work. That was a few months ago and my situation has only gotten worse.
If you do choose an inert gas make sure to test the purity of the tank. I'm pretty sure the argon I ordered wasn't 100 percent. And the exit bag is a pain in the ass to assemble. I used the peaceful handbook for instructions but am not that skilled at arts and crafts. I honestly might not have crafted the exit bag the correct way.
Just know that while the inert gas sounds good, even if it was helium, can go wrong, no matter how reliable everyone reports it to be.
Hopefully this helps